The possibility of using liquid phase instead of vapor phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transitional or variable valence metals, especially cobalt, in a liquid phase of saturated lower aliphatic acid at temperatures from 100.degree. to 320.degree. C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone such as methylethyl ketone or aldehyde such as acetaldehyde. Unfortunately such aldehyde or ketone promoted variable valence metal catalysis was useful only for converting mono-, di- and tri-methylbenzenes to their respective benzene monocarboxylic acids: benzoic, toluic and dimethyl benzoic acids. Two separate, later and somewhat parallel lower temperature (80.degree.-100.degree. C.) modifications of the aldehyde or ketone promoted cobalt catalysis in liquid phase of acetic acid did provide commercially feasible conversion of xylenes to phthalic acids, especially p-xylene to terephthalic acid but only at the expense of using rather high concentrations of cobalt and added quantities of acetaldehyde or methylethyl ketone promoter which were oxidized to acetic acid.
For the liquid phase oxidation of di-, tri- and tetramethylbenzenes with molecular oxygen, it has been discovered that hafnium compounds are unique among compounds of the Group IV metals in being substantially more soluble in solutions of di-, tri-, and tetramethylbenzenes and C.sub.2 to C.sub.6 monocarboxylic acids than compounds of the other Group IV metals. Group IV metals activate Co/Br, Mn/Br or Mn/Co/Br catalysts but hafnium is unique in that hafnium compounds do not precipitate as readily from the liquid phase reaction mixture under reaction conditions or in the presence of reaction products as readily as compounds of other metals of Group IV in the presence of reaction products. Reaction products accordingly are not contaminated by the presence of hafnium compounds.
In reference to the enhancement of catalytic activity by hafnium, there are two aspects. First is the increase in activity of cobalt-bromine, manganese-bromine or manganese-bromine-cobalt catalyst systems by a factor much greater than would be expected by increase in amounts of either manganese and/or cobalt equivalent to the amount of hafnium employed. The second is manifested by the longer sustained initial rapid rate of oxygen consumption when hafnium is a member of the system of catalysis than when the catalyst system comprises cobalt bromine, manganese-bromine or manganese-bromine-cobalt.
Such functions and attributes of hafnium as soluble catalyst compounds in the liquid phase oxidation of di-, tri- and tetramethylbenzenes are indeed unobvious when the character and nature of these functions and attributes of hafnium are considered and compared to manganese and/or cobalt and other Group IV metals. Manganese and cobalt have been known for some time to have the highest oxidation potential of the transition metals characterized in U.S. Pat. No. 2,425,528 as oxidation catalysts. The Group IV metals are not generally considered to be transition metals in a redox system as are cobalt, manganese and other of such metal oxidation catalysts because of their non-variable valence states in oxidation systems.
It is recognized that combinations of cobalt with Group III A or Group IV A metals, as taught in U.S. Pat. No. 3,299,125, are beneficial systems of catalysis for the liquid phase oxidation of alkyl-substituted aromatic hydrocarbons containing two or more alkyl groups which are not adjacent to each other in the ortho position on the aromatic nucleus. Thus the disclosed system of catalysis comprising cobalt and any metal of the Group III A or Group IV A, particularly preferred being scandium, yttrium, neodymium, thorium, zirconium and hafnium, among others, is taught as ineffective for di- and trialkylbenzene such as o-xylene or pseudocumene. Reported yields in U.S. Pat. No. 3,299,125 of terephthalic acid product using a hafnium catalyst range from about 55% to about 76%.
It is also known that, as U.S. Pat. No. 3,562,318 teaches, in the liquid phase oxidation of alkyl-substituted aromatic compounds in the presence of aldehyde or ketone side chain oxidation inhibitors or promoters, beneficial effects are obtained with a cobalt catalyst in combination with one or more metals of the group consisting of Al, Zn, La, Nd, Zr, B or Mg.
It is further known that Canadian Patent No. 1,146,588 teaches a process for preparing aromatic carboxylic acids by oxidation of an alkyl-aromatic hydrocarbon in the liquid phase in the absence of an aliphatic carboxylic acid by means of a gas containing molecular oxygen and in the presence of a catalyst consisting of a soluble cobalt compound and a soluble zirconium or hafnium compound. Atomic ratios of zirconium to cobalt or hafnium to cobalt are ratios lower than 1:5, preferably lower than 1:10. Aromatic carboxylic acids prepared thereby suffer from the drawback of containing many impurities, including aldehydes, alcohols and esters as by-products, which reduces the desirability of the proposed process.
Japanese Patent JP55017348 teaches oxidation of p-tolualdehyde to terephthalic acid in the presence of a metal catalyst selected from a group including zirconium and hafnium and a bromine compound whereby a mineral acid is added to the reaction to effect the oxidation. Also, Japanese Patent JP48096545 teaches a process for oxidizing p-xylene to terephthalic acid in the presence of cobalt and zirconium or hafnium wherein the zirconium or hafnium is present in an amount of from 0.01% to 1% by weight of the cobalt present.
Accordingly, as noted above, the above patents disclose the use of hafnium as a catalyst or co-catalyst in the oxidation of alkyl-substituted aromatic compounds but only with the disadvantage of obtaining relatively low product yields when using hafnium as a catalyst, or product contaminated with many by-product impurities, or of using high concentrations of cobalt promoted with quantities of aldehyde or ketone in the absence of hafnium as a co-catalyst.
The disadvantages of using cobalt in the presence of an aldehyde or ketone or hafnium are overcome by our novel process wherein hafnium is used to activate the cobalt-manganese-bromine catalyst. Our novel process is effective in converting di- or polymethylbenzenes to their corresponding aromatic acids, wherein the mole ratio of hafnium to cobalt, manganese, and bromine is about 1:20 to about 1:600, by weight, preferably, about 1 part hafnium to about 50 parts total cobalt plus manganese plus bromine.
For the liquid phase oxidation of di-, tri and tetramethylbenzenes with molecular oxygen, it has been discovered that hafnium is particularly useful for substantially enhancing the activity of a catalyst system comprising cobalt, manganese and bromine. Acetic acid and/or water-soluble forms of hafnium are especially useful for substantially enhancing the activity of a catalyst system comprising cobalt, manganese and bromine in that acetic acid and/or water-soluble hafnium compounds do not precipitate upon crystallization on di- and polycarboxylic acids obtained from the oxidation reaction, as for example, crystals of terephthalic acid which require further purification before use in preparation of various polymers.
Cobalt is the most expensive component in a cobalt-manganese-bromine catalyst system, approximately ten to fifteen times more expensive than manganese. Therefore, there is great economic incentive to reduce the amount of the oxidation catalyst. Our novel process has succeeded in doing just that by decreasing the concentration of the catalyst components of cobalt and manganese and bromine by a calculated 22%, see Example II. A novel feature of hafnium as a catalyst activator in the oxidation of polymethylbenzenes to the corresponding polycarboxylic acids is that much less catalyst is required to obtain satisfactory yields. Our novel process can also reduce the amount of bromine required, see Example VII. It may also be possible to reduce individually the amount of cobalt and/or manganese required.
Hafnium has been found to be an effective promoter for the cobalt-manganese-bromine catalyst systems for the oxidation of polymethylbenzenes to the corresponding polycarboxylic acids. The term "activation", as used herein, means the ability of a catalyst component to increase the rate of oxidation of polymethylbenzenes to the corresponding polycarboxylic acids. A further novel feature of hafnium as a catalyst activator in the oxidation of polymethylbenzenes to the corresponding polycarboxylic acids is that acetic acid and/or water-soluble compounds of hafnium demonstrate greater acetic acid and/or water solubility under oxidation conditions and/or presence of polycarboxylic acids than do compounds of other elements such as zirconium, molybdenum, vanadium, titanium, and chromium when used in ratios which activate cobalt in the oxidation reaction. Contamination of the resulting polycarboxylic acid product by residual catalyst components is thereby significantly reduced.
Polymethylbenzenes such as o-, m-, and p-xylenes can be oxidized to phthalic acid or phthalic anhydride, isophthalic acid, and terephthalic acid by the process of this invention. Durene can be oxidized to pyromellitic acid or to the pyromellitic anhydrides. Pseudocumene can be oxidized to trimellitic acid or trimellitic anhydride.
Terephthalic acid is produced by a liquid phase oxidation of p-xylene and/or p-toluic acid in a solvent comprising an aliphatic carboxylic acid such as acetic acid. Terephthalic acid is of great commercial importance and is widely used for the production of various polymers, such as fiber-forming polyesters. A process for preparing polyesters of terephthalic acid, particularly polyethylene terephthalate, comprises a direct condensation of terephthalic acid with the respective polyalcohol. For example, terephthalic acid is reacted with ethylene glycol to form bis(.beta.-hydroxyethyl) terephthalate which is then polymerized in a second stage. This direct condensation process is simpler than other known methods such as transesterification of dimethyl terephthalate with the appropriate glycol. However, the direct esterification requires the use of highly purified terephthalic acid. In order to be suitable for the production of polyester fibers, terephthalic acid must be substantially free of any contaminants which lower the melting point of the polyester and/or cause coloration of the polyester. In fact, some impurities which are contained in crude terephthalic acid are color-forming precursors of the terephthalic acid. Additionally, some impurities act as chain terminators in the process to prepare polyesters.
All these impurities have not yet been identified. However 4-carboxybenzaldehyde which is an intermediate oxidation product and which in the following is abbreviated as 4-CBA, generally is found in crude terephthalic acid. It is known that the degree to which coloration in the polyester is induced is less if the 4-CBA content of the terephthalic acid is low. While pure 4-CBA itself does not necessarily promote coloring during polymerization, this impurity is a convenient tracer for evaluating the degree to which terephthalic acid has been refined. A process which can reduce the 4-CBA content of terephthalic acid reduces also the content of color-forming precursors.
From U.S. Pat. No. 3,584,039 issued to Delbert H. Meyer, incorporated by reference, it is known that fiber-grade terephthalic acid may be prepared by purifying crude terephthalic acid by means of a reduction procedure. The process is essentially comprised of treating an aqueous solution of crude terephthalic acid with hydrogen in the presence of a supported or unsupported Group VIII metal catalyst, whereby the metal and the support are insoluble in the solution under the working conditions. By this process, the amounts of 4-CBA and other coloring impurities contained in terephthalic acid are reduced by formation of removable products. Purified terephthalic acid is then recovered by crystallization, filtration to recover the crystalline product, and drying.
As noted above, the oxidation of p-xylene can be in the presence of a hafnium compound as a co-catalyst for the cobalt-manganese-bromine catalyst. However, it is considered that hafnium may also act as a catalyst poison to the supported or unsupported Group VIII metal catalyst, typically palladium, useful in the reduction procedure to prepare fiber-grade terephthalic acid. Additionally, it is also known that the aliphatic carboxylic acid which comprises the solvent for the liquid phase oxidation of p-xylene and/or p-toluic acid can act as a poison for the reduction catalyst. Methods have been proposed for replacement or extraction of the aliphatic carboxylic acid, such as acetic acid, from the oxidation effluent with water. For example, U.S. Pat. No. 3,839,436 teaches contacting an oxidation slurry with water wherein water is introduced into the bottom of a displacement zone to contact the oxidation effluent in a vertical chamber to effect precipitation of the product acid through the column of water and to remove an aqueous slurry suitable for catalytic purification from the bottom of the column. Concurrent removal of at least a portion of the hafnium component of the soluble hafnium cobalt-manganese-bromine oxidation catalyst by extraction with water can therefore result.
Water extraction of aliphatic carboxylic acids such as acetic acid and a co-catalyst such as hafnium can therefore comprise a positive method to reduce the concentration of acetic acid and hafnium in the crude terephthalic acid.
It is therefore an object of this invention to provide a co-catalyst to reduce the amount of cobalt component in the oxidation catalyst. It is a further object of this invention to provide a hafnium co-catalyst in the oxidation of polymethylbenzenes to the corresponding polycarboxylic acids wherein much less cobalt is required to obtain satisfactory yields. Moreover, it is an object of this invention to provide a hafnium co-catalyst which is sufficiently acetic acid and/or water soluble to be at least partially removed or extracted from crystals of crude terephthalic acid before the crude terephthalic acid is slurried in water for a reduction process.